1. Field of the Invention
The present invention relates to aircraft approach and landing systems for moving platforms. The present invention also relates to systems based on the Global Positioning System Kinematic Carrier Phase Tracking technology, and to systems that determine relative position solutions between two moving platforms.
2. Related Art
Landing an aircraft on a moving platform or landing area creates significant problems for approach and landing systems. For example, in naval applications such as landing an aircraft on an aircraft carrier, aircraft landing operations require a robust and precise navigation system for auto-coupled approaches and landings. A radar based system called the SPN-46 system has been previously developed to provide the required sensor accuracy for auto-coupled approaches for approach and landing control for an aircraft landing on an aircraft carrier. The SPN-46 system, however, tends not to be reliable during limited visibility conditions, such as those caused by precipitation, and it is in such limited visibility conditions that a system, such as the SPN-46, is most required.
Approach and landing systems, based upon the Global Positioning System, do not suffer from this disadvantage and are operable in bad weather. The Global Positioning System ("GPS") presently consists of a constellation of 24 satellites that continuously broadcasts time and frequency data on two frequencies, L1 and L2. GPS receivers have been developed to correlate these time and frequency measurements to develop range measurements. Using these range measurements and triangulating, a determination of the GPS receiver's position relative to a point on the earth may be determined. The most common method utilized is to determine a position relative to a known surveyed reference point that is fixed on the earth's surface. In the context of approach and landing of aircraft, this reference point is typically some stationary point on the ground near the landing area, such as a touchdown point for the aircraft. To implement a GPS landing system, algorithms have been developed that use GPS data to produce an accurate known position relative to a surveyed aircraft touchdown point. One such algorithm is known as a Kinematic Carrier Phase Tracking (KCPT) solution.
For aircraft carrier landing systems, however, the landing area is not stationary, and there is no surveyed touchdown point. In addition, Navy requirements specify that the aircraft position must be known relative to the touchdown point on the aircraft carrier to an accuracy of one foot. Because the touchdown point on an aircraft carrier is constantly changing, along with other environmental aspects of the aircraft carrier that are hostile to the KCPT solution, existing KCPT solutions do not provide adequate results in an aircraft carrier application. In addition, to operate a KCPT solution on an aircraft carrier, the process must be able to accommodate multiple maskings and cycle slips on a moving platform.
In other applications involving the tracking of one moving object with respect to another moving object, similar problems are encountered with traditional systems relying upon GPS technology. Because there is no surveyed point to use as a reference, the range and direction solutions are difficult to determine.
What is needed is a system for determining an accurate relative solution vector for environments between two moving platforms, such as in aircraft carrier approach and landing operations.